Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For instance, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed.
Radially expandable stents come in a variety of different configurations to provide optimal performance to various different particular circumstances. For instance, the patents to Lau (U.S. Pat. Nos. 5,514,154, 5,421,955, and 5,242,399), Baracci (U.S. Pat. No. No. 5,531,741), Gaterud (U.S. Pat. No. 5,522,882), Gianturco (U.S. Pat. Nos. 5,507,771 and 5,314,444), Termin (U.S. Pat. No. 5,496,277), Lane (U.S. Pat. No. 5,494,029), Maeda (U.S. Pat. No. 5,507,767), Marin (U.S. Pat. No. 5,443,477), Khosravi (U.S. Pat. No. 5,441,515), Jessen (U.S. Pat. No. 5,425,739), Hickle (U.S. Pat. No. 5,139,480), Schatz (U.S. Pat. No. 5,195,984), Fordenbacher (U.S. Pat. No. 5,549,662) and Wiktor (U.S. Pat. No. 5,133,732), each include some form of radially expandable stent for implantation into a body lumen.
Radially expandable surgical stents are known in the art which are formed from a single helical element and which exhibit desirable radial expandability characteristics and also readily flex when passed along a curving body lumen. One such stent is described in U.S. Pat. No. 5,591,230 to Horn. While the Horn stent exhibits desirable flexibility and radial expandability characteristics it is subject to axial contraction when compressive loads are experienced. For instance, when the Horn prior art stent is in a radially collapsed configuration and is passing through a body lumen, if leading portions of the stent temporarily catch against walls of the body lumen the trailing portions of the stent can be caused to collapse axially toward the leading edge a significant amount.
Such axial contraction resulting from axial loads is referred to as an accordion effect. This axial contraction is distinct from axial contraction exhibited by many stents when they are being radially expanded within a body lumen but under no axially compressive load. This accordion effect is undesirable in that it can make the stent more difficult to position precisely within the body lumen.
Additionally, if the accordion effect occurs to a great extent or while the stent is flexed and has a central axis thereof bent, there is an opportunity for adjacent turns in the helical element to bind together. This binding can sometimes cause breakage of the stent elements or permanent deformation of the stent in an undesirable manner. Accordingly, a need exists for a single helical element surgical stent which resists the accordion effect while still being radially expandable and while still having sufficient flexibility to follow tortuous pathways within a body lumen.